stream: optimize webstreams pipeTo further

[MattiasBuelens]

I've started looking more closely at the pipeTo() implementation, and found some low-hanging fruit to further optimize it:

• We don't have to await writer.ready every time, that's only needed if there is backpressure. So we check that first.
• We don't have to go through writer.desiredSize to check if there is backpressure, we can read that directly from the internal state of the WritableStreamDefaultController.
• readableStreamDefaultControllerCallPullIfNeeded already checks state === 'readable' && !closeRequested through readableStreamDefaultControllerCanCloseOrEnqueue, so we don't have to repeat that check.

This also fixes a few more edge cases:

• Previously, we were using writer.desiredSize inside pipeTo(), which is part of the public WritableStreamDefaultWriter API, and step 15 of the spec explicitly forbids using the public API since it can be modified by user-land code. We now bypass this entirely using dest[kState].backpressure.
  • There doesn't seem to be much WPT test coverage for this... I might add more tests to the upstream patched-global.any.js suite.
• If the writable end becomes closed or errored in the middle of a batch of synchronous reads, we should stop immediately.

On my machine, this improves the webstreams/pipe-to.js benchmark by 3% to 7%:

Baseline @ 199daab

webstreams\pipe-to.js highWaterMarkW=512 highWaterMarkR=512 n=500000: 1,639,426.502378644
webstreams\pipe-to.js highWaterMarkW=1024 highWaterMarkR=512 n=500000: 1,672,850.078172284
webstreams\pipe-to.js highWaterMarkW=2048 highWaterMarkR=512 n=500000: 1,684,703.098034221
webstreams\pipe-to.js highWaterMarkW=4096 highWaterMarkR=512 n=500000: 1,658,174.6018059512
webstreams\pipe-to.js highWaterMarkW=512 highWaterMarkR=1024 n=500000: 1,684,320.5922340695
webstreams\pipe-to.js highWaterMarkW=1024 highWaterMarkR=1024 n=500000: 1,663,948.3297453062
webstreams\pipe-to.js highWaterMarkW=2048 highWaterMarkR=1024 n=500000: 1,651,671.1443471392
webstreams\pipe-to.js highWaterMarkW=4096 highWaterMarkR=1024 n=500000: 1,630,100.2576862487
webstreams\pipe-to.js highWaterMarkW=512 highWaterMarkR=2048 n=500000: 1,662,200.8869503932
webstreams\pipe-to.js highWaterMarkW=1024 highWaterMarkR=2048 n=500000: 1,638,533.5910855907
webstreams\pipe-to.js highWaterMarkW=2048 highWaterMarkR=2048 n=500000: 1,671,184.435256644
webstreams\pipe-to.js highWaterMarkW=4096 highWaterMarkR=2048 n=500000: 1,639,392.637815542
webstreams\pipe-to.js highWaterMarkW=512 highWaterMarkR=4096 n=500000: 1,644,874.8168020674
webstreams\pipe-to.js highWaterMarkW=1024 highWaterMarkR=4096 n=500000: 1,702,132.8746198711
webstreams\pipe-to.js highWaterMarkW=2048 highWaterMarkR=4096 n=500000: 1,608,151.915035545
webstreams\pipe-to.js highWaterMarkW=4096 highWaterMarkR=4096 n=500000: 1,688,220.1736233155

This PR @ 7b31401

webstreams\pipe-to.js highWaterMarkW=512 highWaterMarkR=512 n=500000: 1,768,254.040106831
webstreams\pipe-to.js highWaterMarkW=1024 highWaterMarkR=512 n=500000: 1,717,563.2174905646
webstreams\pipe-to.js highWaterMarkW=2048 highWaterMarkR=512 n=500000: 1,746,963.515365767
webstreams\pipe-to.js highWaterMarkW=4096 highWaterMarkR=512 n=500000: 1,787,073.738236587
webstreams\pipe-to.js highWaterMarkW=512 highWaterMarkR=1024 n=500000: 1,756,757.2790359478
webstreams\pipe-to.js highWaterMarkW=1024 highWaterMarkR=1024 n=500000: 1,704,454.421184323
webstreams\pipe-to.js highWaterMarkW=2048 highWaterMarkR=1024 n=500000: 1,761,150.6372019118
webstreams\pipe-to.js highWaterMarkW=4096 highWaterMarkR=1024 n=500000: 1,766,948.0354718352
webstreams\pipe-to.js highWaterMarkW=512 highWaterMarkR=2048 n=500000: 1,727,179.5537866165
webstreams\pipe-to.js highWaterMarkW=1024 highWaterMarkR=2048 n=500000: 1,768,980.3632565797
webstreams\pipe-to.js highWaterMarkW=2048 highWaterMarkR=2048 n=500000: 1,752,746.9927243474
webstreams\pipe-to.js highWaterMarkW=4096 highWaterMarkR=2048 n=500000: 1,793,850.0362895865
webstreams\pipe-to.js highWaterMarkW=512 highWaterMarkR=4096 n=500000: 1,769,294.5999359516
webstreams\pipe-to.js highWaterMarkW=1024 highWaterMarkR=4096 n=500000: 1,768,876.4768791925
webstreams\pipe-to.js highWaterMarkW=2048 highWaterMarkR=4096 n=500000: 1,748,478.5613798152
webstreams\pipe-to.js highWaterMarkW=4096 highWaterMarkR=4096 n=500000: 1,783,529.4621231847